Recently, with the change into a knowledge-based society, where information resources are important, and the increase in data volume used by individuals, the need for data storage devices has been ever increasing. Also, with the rapid development and the increasing demand in fields such as large volume data storage devices, information appliance devices, personal portable information devices, and digital image devices, requiring data storage, the demand for ultra small, high density nonvolatile data storage media is increasing. Conventional hard disks have difficulties in miniaturization and are less feasible in high density integration due to their limitations due to the ultra-paramagnetic feature, which is a unstable data stage caused by thermal energy. Meanwhile, at present, with flash memories, it is difficult for the integration density to reach the data storage capacity of hard disks, and CDs and DVDs representing optical data storage devices can not overcome their limitation due to the light wavelength, which is required in response to a high degree of integration. Thus, as an alternative, recently, various kinds of scanning probe type data storage devices and storage media are being studied. Scanning probe type data storage devices are meant by devices utilizing Atomic Force Microscopes (AFM), which use forces between atoms, and Scanning Probe Microscopes (SPM) such as Scanning Tunneling Microscopes (STM).
For the Scanning Probe Microscope type data storage devices, studies have been made concerning thermal recording type polymers, magnetic recording type ferromagnetic materials, organic thin films or phase transition materials using change of electric conductivity according to phase transition, and static electric force type ferroelectric media using surface charge change, as data storage media. As methods for writing/reading such a storage medium, one method is the use of electrical/magnetic forces, which forms domains through reversal of electric/magnetic polarization of ferroelectric or ferromagnetic material by applying an external electric/magnetic field on a scanning probe, and another method is a thermal method, that is, the forming of well-shaped deformations or changing the electric conductivity on the surface of a polymer by applying heat on the storage medium through a probe. The method of thermal writing/reading is disadvantageous in that it has a low speed of operation, a limited number of times that the writing can be repeated, and high power consumption. Also, the method of electric conductivity change is disadvantageous due to the oxidation of the storage medium and abrasion of the probe.
Thus, as a method for overcoming such problems and disadvantages, the use of ferroelectric storage media is being actively studied. The principle of writing a data storage medium by using a ferroelectric storage medium is as follows. A probe, instead of an upper electrode, contacts the ferroelectric storage medium, which is formed on a lower electrode, so as to form a capacitor structure. Then, by applying an external voltage on the probe, the electric polarization of the ferroelectric thin film is changed toward the direction of the electric field. Thus, by placing the probe on a predetermined point of the surface of the ferroelectric thin film, and by applying a voltage on this point, a domain (data bit) can be formed by arranging electric polarization of a ferroelectric material into a predetermined direction, enabling a data write operation. Also, if an AC voltage having uniform frequency is applied between the probe and the lower electrode, the amplitude of the ferroelectric thin film is changed by the electric force among the electric polarization of the ferroelectric material in accordance with the applied AC voltage, and thus such change of the amplitude changes the minute force between the probe and the ferroelectric thin films, and the magnitude of such change differs according to the direction of the domain. Therefore, data can be read by analyzing it. If a ferroelectric thin film is used for a storage medium, it will have advantages of high speed of data read/write, low power consumption, and ability of repeated writing. However, there is much to be done for solving the problems of this technology.
At present, as ferroelectric materials, peroveskite oxides having the crystal structure of ABO3 are being studied. The representing materials thereof include BaTiO3, (Pb,Zr)TiO3(PZT), and LiTaO3. The peroveskite oxide having the crystal structure of ABO3 has an isotropic cubic crystal structure showing the six directional electric polarizations upward, downward, leftward, rightward, frontward and backward by minute displacement of ions in the peroveskite oxide. Therefore, eventually its electric polarizations are displaced to have 90 degree directional differences with each other. Therefore, it has a 90 degree domain structure. Such 90 degree domain structure generates mechanical strains between 90 degree domains and makes its electric polarization unstable, and as a result, it has been difficult to fabricate a ferroelectric storage medium including the 90 degree domain structure in nano-scale as a unit bit for data storage. Also, if the electric polarization has a 90 degree domain structure, the long term stability of electric polarization in the domain will be decreased due to strains therebetween, so the remaining polarization in the structure becomes gradually decreased, which would result in disappearance of long term ability of data storage and errors in read operation of the stored data. That has been the great obstacle in industrialization of the ferroelectric data storage medium. Furthermore, as a thin film type storage medium is fabricated in nano-scale thickness, its electric polarization becomes unstable due to the finite size effect, lowering stability of the storage medium.
Recently, it was reported that at least a 20 nm scale bit can be produced by uniformly arranging the direction of polarization in the material through epitaxial growth of a PZT thin film, being capable of storing tens of Gbit/cm2 data (Applied Physics Letters, P. Paruch et al., Vol 79, 530 (2001)). However, there is a difficulty in forming a bit because poly domains including 90 degree domains can be easily formed due to lattice strains between a substrate and a thin film during the epitaxial growth of the PZT thin film. Also, it requires a process for fabricating a thin film in atomic scale with a high degree of surface flatness, and thus it still has a problem of retaining time in data storage.
In one effort to overcome the above problems, Cho group in Japan recently reported that an at least 8 nm scale bit with very small thickness, which enables 10 Tbit/inch2 of data storing ability when analyzed by a Scanning Nonlinear Dielectric Microscope (SNDM), is formed from a layer of LiTaO3 single crystals by using mechanical chemical polishing. However, this technology (mechanical chemical polishing) requires a very delicate process of fabricating single crystals into a very thin layer.